I'm having difficulty with Javascript instance variables. I'm trying to make an easy way to refresh the chat page, looking for new messages. The AJAX call supplies a mid, or the lowest message id to be returned. This allows the call to only ask for the most recent messages, instead of all of them.

MessageRefresher.prototype._latest_mid;

function MessageRefresher(latest_mid) {
    this._latest_mid = latest_mid; // it thinks `this` refers to the MessageRefresher object
}

MessageRefresher.prototype.refresh = function () {
    refreshMessages(this._latest_mid); // it thinks `this` refers to the window
    this._latest_mid++;
}

function refreshMessages(latest_mid) {
    $.getJSON('API/read_messages', { room_id: $.getUrlVar('key'), mid: latest_mid }, function (messages) {
        for (var i = 0; i < messages[0].length; i++) {
            var newChild = sprintf("<li>%s: %s</li>", messages[1][i], messages[0][i]);
            $("#messages").append(newChild);
        }
    });

    var messageRefresher = new MessageRefresher(0);
    setInterval(messageRefresher.refresh, 1000);

This results in all the messages being printed out, over and over again.

I know it has other bugs, but the main issue I'm trying to work out right now is the use of the instance variable. Or is there another way I should be going about doing this?